Colorado Aspen Trees “Starving to Death”

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“When the trees are stressed by heat and dry conditions, the stomatae
(small openings in the leaves) close. That slows the loss of water in
response to drought but also slows photosynthesis, the process by
which plants create energy.

SUMMIT COUNTY – Evidence is growing that drought conditions are
killing Colorado’s aspens at an unprecedented rate.

More than 56,000 acres of aspens have recently died in the state,
according to a paper published by a group of Forest Service
scientists last year. Nearly 10 percent of the aspen stands in the
San Juan National Forest have been affected, witth mortality
increasing at a rapid rate.

An intense drought in the early 2000s was the likely trigger for the
startling decline in the health and vigor of one of Colorado’s
signature trees, said James Worrall, one of the primary authors of
the study.

“I didn’t feel comfortable making the direct link between climate
change and aspen decline,” Worrall said. “But it’s safe to say, if
the climate change predictions turn out to be true, we’re going to
see more aspen problems,” Worrall said. The tree’s range could shrink
significantly, especially on south-facing slopes.

It’s conceivable, but not likely, that aspens could spread to new
areas in response to climate change, Worrall said. Limiting factors
would be soil conditions, as well as the presence of existing aspen
stands. The trees rarely sprout from seeds. Almost all reproduction
comes from new chutes growing from health root clusters, so any
spread would be very slow.

Other instances of aspen mortality, for example in the Great Lakes
region, have also been tied to warm and dry conditions, according to
the report.

Another large-scale die-back was documented in the 1980s and 1990s in
the prarie provinces of Alberta and Saskatchewan, associated with the
dual stresses of drought and insect defoliation, followed by
secondary wood-boring insects and diseases.
And in the early 1970s, a similar trend was observed in Utah and
Wyoming, at the time linked to fire suppression and overgrazing by
deer and elk.

But the rapid spread of the current aspen decline in southern
Colorado appears to be unprecedented. Aspen mortality in one part of
the San Juan National Forest increased 58 percent from 2005 to 2006,
with a five-fold increase in the incidence of mortality over a three
to four year period.

As the scientists studied aspen stands in four southern Colorado
national forests, they also found that there is very little
regeneration of aspen growth. The trees generally spread via suckers,
or shoots of the roots of healthy trees. After several years of
drought, the aspens are basically starving to death, Worrall said.

Worrall explained the physiology of the die-off: “The stress we’re
seeing to the overstory is an energy drain that lead to poor root
conditions. They don’t have the energy to re-grow suckers.”

When the trees are stressed by heat and dry conditions, the stomatae
(small openings in the leaves) close. That slows the loss of water in
response to drought but also slows photosynthesis, the process by
which plants create energy.

The researchers also found that agents that typically attack and kill
healthy aspen stands in Colorado were not significant factors in the
recent dramatic die-back.

The recent mortality in southwestern Colorado had ” a sudden onset
and was very rapid,” in contrast to previous documented episodes of
“aspen decline,” according to the report. And the mortality agents
appear to be different, suggesting that climate factors are involved.